Used vegetable oil can be turned into bio diesel via chemical reaction. This chemical conversion is called transesterification. [definition from wikipedia - In organic chemistry, transesterification is the process of exchanging the organic group R" of an ester with the organic group R' of an alcohol. These reactions are often catalyzed by the addition of an acid or base.]
We can produce biodiesel from used vegetable oil by a process involves reacting vegetable oils or animal fats catalytically with a short-chain aliphatic alcohols (typically methanol or ethanol).
This process can simplified then it will boost in small and medium scale business and even as home made biodiesel projects.
Following readings will helpful for more understanding.